Production of metal articles with holes in them



3. KEEGAN Dec. 30, 1958 PRODUCTION OF METAL ARTICLES WITH HOLES IN THEMFiled Sept. 3, 1957 E TC'HED ass 707maoae Gm.

UNE TBHE D LEA CHI/VG TIME IN HO UB8 United States Patent PRODUCTION OFMETAL ARTICLES WITH HOLES IN THEM Application September 3, 1957, SerialNo. 681,807

Claims priority, application Great Britain September 12, 1956 Claims.(Cl. 1484) The present invention relates to an improvement in theprocess of leaching filler material from, hot-worked, heat-resistantmetal articles having at least one internal filled passage and, moreparticularly, to the leaching of ferrous filler materials from saidarticles.

It is well known that one method of producing metal articles with holesor internal passages in them is first to produce a billet or the likewith one or more holes in it; insert a filler in each hole; then toshape the billet or the like by extrusion or otherwise, the filler beingsimultaneously distorted; and finally to remove the filler. Usually thefiller, and therefore the hole containing it, is considerably elongatedand reduced in cross-sectional area during the shaping step. This methodcan be applied to the production of hollow turbine blades fromnickel-chromium heat-resisting alloys, a billet of the alloy beingdrilled to form circular holes into which a ferrous filler is inserted;thereafter the billet is extruded to airfoil shape.

The usual method of removing the filler is to leach it out of the holesby an acid solution which attacks the filler but not the metal of whichthe article is made, or at least attacks the filler preferentially.Particularly advantageous types of ferrous fillers which may be used inconjunction with hot-workable heat-resistant alloys are described andclaimed in U. S. patent applications Serial Nos. 509,380 and 472,755.These ferrous fillers may be leached by means of dilute solutions ofcommon mineral acids which do not substantially attack theheat-resistant metal. Ferritic alloys of iron, manganese and titaniumcontaining about 5% to about manganese, about 1% to about 10% titaniumwith the balance essentially iron and iron-ceramic mixtures, forexample, iron and magnesia mixtures containing from about 5% to aboutmag nesia with the balance essentially a continuous phase of iron, areparticularly suitable.

As was mentioned hereinbefore, these ferrous fillers may be leached withdilute aqueous solution of common mineral acids. However, the leachingoperation, even at elevated temperatures close to the boiling point ofthe acid solution, proceeds very slowly.

Although many attempts were made to overcome the foregoing diflicultiesand other disadvantages, none, as far as I am aware, was entirelysuccessful when carried into practice commercially on an industrialscale.

It has now been discovered that the leaching of ferrous fillers fromfilled metal articles made from heat-resistant hot-workable alloys maybe substantially accelerated by means of a special pretreatment prior tothe leaching operation.

It is an object of the present invention to provide an improved processfor leaching ferrous filler from filled heatresistant hot-workable metalarticles.

Another object of the present invention is to provide an improvedprocess whereby the leaching of ferrous filler from filled metal objectsmade of hot-workable heat-resistant metal objects may be accelerated.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the accompanying drawing in which:

Figure 1 is a histogram depicting the relationship between the depth offiller leached and time, as measured on similar articles, leached in asimilar manner and in the same reagent, one of which has been subjectedto the novel pretreatment of the present invention and the other ofwhich has been treated according to prior practice.

Generally speaking, the present invention contemplates the use of aconditioning pretreatment step comprising the chemical removal inaqueous media of the surface layer of the heat-resistant filled metalarticle prior to leaching with the conventional leaching reagents.Ascording to this invention, the oxide layer is removed before theleaching by subjecting the articles to an acidetching treatment in anaqueous acidic solution which attacks both the filler and the metal ofwhich the article is made. The filler can then be leached by a solutionwhich only, or preferentially (or selectively), attacks the filler. Theacid-etching treatment may be carried out merely by immersion of thearticle in an acidic solution or by immersion of the article in saidacidic solution coupled with anodic electrolytic action. Satisfactoryetching solutions or reagents include aqueous solutions of hydrochloricacid, sulphuric acid and nitric acid which contain between about 50% toabout 60% free concentrated acid. The etching solution should bemaintained between about C. to about 102 C. when no electrolytic actionis involved. On the other hand, when electro-etching is used thetemperature should be maintained between about 20 C. and about 25 C. Asa general rule, the article should remain in the bath for between about10 and 20 minutes. However, the etching should be carried on only forsuflicient time for the oxide layer to be removed, or the article may bedamaged.

The preferred etching solution for an article made of a nickel-chromiumheat-resisting alloy is one consisting of equal parts of water andcommercial concentrated hydrochloric acid; this solution should be usedat its boiling point, when the etching is carried out by immersion only,but can be used at room temperature when the etching is electrolytic.Electrolytic treatment in a 6 volt circuit is satisfactory, the articleforming the anode. It has been found that treatment of articles in thissolution in either manner for 15 minutes is, in most cases, suflicientto remove the oxide layers but insuificient to do appreciable damage tothe article. Preferably, the filler should be leached from the articlesas soon as possible after the surface layer has been removed and in anyevent before the oxide layer has had time to reform. Suitable leachingsolutions are described in the aforementioned patent applications.Aqueous nitric acid solutions and mixtures thereof with other mineralacids or salts are especially suitable.

Referring now to the drawing, wherein the width of the bars representsfour-hour increments in time, and the length of the bars representsincrements in depth of filler leached, it is to be observed that thehistograms show the dramatic improvement in leaching action when theprocess of the present invention'is used. In the histogram illustratingleaching after the etching treatment of the present invention, it is tobe observed that a total of 16 hours was required to leach a total of203.2 centimeters (cms.) of filler from turbine blade blanks whereas toleach the same depth of filler in an identical turbine blade blank atotal of 24 hours was required.

For the purpose of giving those skilled in the art a betterunderstanding of the invention and/or a better appreciation of theadvantages of the invention, the following illustrative example isgiven:

3 EXAMPLE I A series of sirnliar turbine blades, made from heatresistantnickel-chromium alloy containing 18-21% chromium, 1.s-a% titanium, =0.8-.2.% aluminum, with the balance nickel, havingcsmall internalpassagesrfilled with a..ferr.o.us filler-having l. atcomposition .of 10%manganese, 2% titanium. balance iron,were produced. All samples ofthese-blades were gritwblasted. One set of blades was etchedfor l5minUtesJin 50% commercial concentrated hydrochloric: acidmaintainediatwthe boiling point. The other; set of blades was given nopretreatment. Two different leaching solutions were used for both sets.Leachingsolutionnumber .1 comprised an aqueous solution .of %commercialconcentrated nitric acid and 1% commercialuconcentrated. hydrochloricacid by volume. Aqueous leaching solution number 2 contained 25%commercial concentrated nitric acid. The advantages provided by thepresent invention are illustrated by the following tableiin whichleaching time indicates the time toremove allthe filler material:

In a further series of tests it was found that without the etchingtreatment according to this invention the filler was removed from holesto adepth of 55 cms. in 4 hours when leached with nitric-hydrochloricacid solution, whereas after treatment according to the invention thefiller wasremoved to a depth of 85 cms. in the same time.

In yet another set of experiments using similar turbine blades havingholes 13.2 cms. deep it took 16 hours to remove the filler after theacid etching treatment of this invention: but 24 hours without it. Withthe etching pretreatment at the fourth :hour 5.2 cms. were leached andat the eighth hour 9.8.cms. were leached. Without the etchingpretreatment ,of .the present invention at the fourth hour 3.6 cms. wereleached and at the eighth hour 7.6 cms. were leached.

With respect to the manufacture of turbine blades using thenovelleaching process of the present invention, it should be understoodthat the term heat-resistant metal is used to include austeniticnickel-chromium, nickel-chromium-iron, cobalt-nickel-chromium,cobaltchromium and cobalt-chromium-iron alloys which contain at leastabout 25% nickel plus chromium, cobalt plus chromium or nickel pluschromium plus cobalt in addition to small amounts of aluminum, titanium,molybdenum, tungsten, niobium, tantalum, silicon, manganese, zirconiumand boron which may optionally be present in the alloys.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. A process for producinghollow metal articles from heat-resistantmetal which comprises in combination the steps ofhot working a billet ofheat-resistant metal filled with a ferrous filler, pretreating theformed heat-resistant metal containing said ferrous filler by subjectingit to a chemical etching treatment in an aqueous acidic medium to removethe surface layer therefrom, and thereafter removing said ferrous fillerfrom said formed heatresistant metal by selectively leaching saidferrous filler in an acidic medium.

2. A process for producing hollow metal articles from heat-resistantmetal which comprises in combination the steps of hot working a billetof heat-resistant metal filled wither ferrous filler, pretreating theformed heat-resistant metal containing said ferrous filler by subjectingit to a chemical .etching treatment in an aqueous solution of acidselected from the group consisting of sulphuric acid, hydrochloricacidandnitricacid, which contains between about to about freeconcentrated acid, to remove the surface layer from said formedheat-resistant metal :and thereafter removing said ferrous filler fromsaid-formed heat-resistant metal by selectively leaching said'ferrousfiller in an acidic medium.

3. A process as set forth and described in claim 2 wherein the aqueousacidic solution is maintained at a temperature between about C. andabout 102 C., and the formed heat-resistant metal is treated therein fora timebetween about 10 and 20 minutes.

4. Aaprocess forproducing hollow metal articles from heat-resistantmetal which comprises in combination the steps of'hot working a billetof heat-resistant metal filled withaferrous filler, pretreating theformed heat-resistant metal containing said ferrous filler by subjectingit to a chemical etching treatment for about 15 minutes in anaqueoussolution containing between about 50% to about 60%freeconcentrated hydrochloric acid to remove the surface layertherefrom, and thereafter removing said ferrous filler fromsaid formedheat-resistant metal by selectively leaching said ferrous filler in anacidic medium.

'5. A'processfor producing hollow metal articles fromheat-resistantmetal which comprises in combination the steps of hotworking a billet of heat-resistant metal filled with a ferrous filler,pretreating the formed heat-resistant metal containing said ferrousfiller by subjecting it to an electrochemical'etching treatment in anaqueous solution of acid selected'from the group consisting of sulphuricacid, hydrochloric acid and nitric acid, which contains between about50% to about 60% free concentrated acid and which ismaintained at atemperature between about 20 C. to about 25 C.,-to remove the surfacefrom said heat-resistantxmetal, and thereafter removing said ferrousfiller from said formed heat-resistant metal by selectively leachingsaid ferrous filler in an acidic medium.

ReferencesCited in the tile of this patent UNITED. STATES PATENTS2,047,555 Gardner July 14, 1936

2. A PROCESS FOR PRODUCING HALLOW METAL ARTICLES FROM HEAT-RESISTANTMETAL WHICH COMPRISES IN COMBINATION THE STEPS OF HOT WORKING A BILLETOF HEAT-RESISTANT METAL FILLED WITH A FERROUS FILLER, PRETREATING THEFORMED HEAT-RESISTANT METAL CONTAINING SAID FERROUS FILLER BY SUBJECTINGIT TO A CHEMICAL ETCHING TREATMENT IN AN AQUEOUS SOLUTION OF ACIDSELECTED FROM THE GROUP CONSISTING OF SULPHURIC ACID, HYDROCHLORIC ACIDAND NITRIC ACID, WHICH CONTAINS BETWEEN ABOUT 50% TO ABOUT 60% FREECONCENTRATED ACID, TO REMOVE THE SURFACE LAYER FROM SAID FORMEDHEAT-RESISTANT METAL AND THEREAFTER REMOVING SAID FERROUS FILLER FROMSAID FORMED HEAT-RESISTANT METAL BY SELECTIVELY LEACHING SAID FERROUSFILLER IN AN ACIDIC MEDIUM.